For aluminum alloys, it is generally accepted that fatigue crack growth resistance under spectrum loading generally decreases as strength increases, and vice-versa. See, e.g., Wanhill et al., “Flight simulation and constant amplitude fatigue crack growth in aluminum-lithium sheet and plate”, International Congress on Aeronautical Fatigue, Tokyo, Japan, 1991. Additionally, some 7xxx alloys, such as AA7150 or AA7055, realize higher strengths than 2xxx alloys, such as AA2024, AA2026 and AA2099, but also realize significantly lower fatigue crack growth resistance.